Masking effects in binaural detection and interaural time discrimination

This study was designed to investigate the effects of masker level and frequency on binaural detection and interaural time discrimination. Detection and interaural time discrimination of a 700-Hz sinusoidal signal were measured as a function of the center frequency and level of a narrow-band masking noise. The masker was a continuous, diotic, 80-Hz-wide noise that varied in center frequency from 250 to 1370 Hz. In the detection experiment, the signal was presented either diotically (NoSo) or interaurally phase reversed (NoS pi). In the interaural time discrimination experiment, the signal level needed to discriminate a 30-microseconds interaural delay was measured. As would be expected, the presence of the masker has a greater effect on NoSo detection than NoS pi detection, and for masker frequencies at or near the signal frequency. In contrast, interaural time discrimination can be improved by the presence of a low-level masker. Also, performance improves more rapidly as the signal/masker frequency separation increases for NoSo detection than for interaural time discrimination and NoS pi detection. For all three tasks, significant upward spread of masking occurs only at the highest masker level; at low masker levels, there is a tendency toward downward spread of masking.     

Effects of level and background noise on interaural time difference discrimination for transposed stimuli

A method-of-adjustment procedure was used to measure thresholds for detecting a continuous sequence of brief 2-kHz tonal pulses in the presence of random-frequency masking sequences. Masker pulses consisted of either one or eight sinusoidal components and were either synchronous or asynchronous with the signal pulses. Effects of pulse rate and asynchronous gating were generally consistent with a reduction in informational masking due to segregation of the signal and masker streams. Despite use of continuous stimulus presentation to encourage stream segregation, masking was still obtained from most listeners in most conditions.     

Monaural and interaural intensity discrimination: level effects and the "binaural advantage"

This study examined whether the level effects seen in monaural intensity discrimination (Weber's law and the "near miss") in a two-interval task are also observed in discrimination of interaural intensity differences (IIDs) in a single-interval task. Both tasks were performed for various standard levels of 4-kHz pure tones and broadband noise. The Weber functions (10 log deltaI/I versus I in dB) in the monaural and binaural conditions were parallel. For noise, the Weber functions had slopes close to zero (Weber's law) while the Weber functions for the tones had a mean slope of -0.089 (near miss). The near miss for the monaural and binaural tasks with tones was eliminated when a high-pass masker was gated with the listening intervals. The near-miss was also observed for 250- and 1000-Hz tones in the binaural task despite overall decreased sensitivity to changes in IID at 1000 Hz. The binaural thresholds showed a small (about 2-dB) advantage over monaural thresholds only in the broadband noise conditions. More important, however, is the fact that the level effects seen monaurally are also seen binaurally. This suggests that the basic mechanisms responsible for Weber's law and the near miss are common to monaural and binaural processing.     

The effect of head-induced interaural time and level differences on speech intelligibility in noise

A study was made of the effect of interaural time delay (ITD) and acoustic headshadow on binaural speech intelligibility in noise. A free-field condition was simulated by presenting recordings, made with a KEMAR manikin in an anechoic room, through earphones. Recordings were made of speech, reproduced in front of the manikin, and of noise, emanating from seven angles in the azimuthal plane, ranging from 0 degree (frontal) to 180 degrees in steps of 30 degrees. From this noise, two signals were derived, one containing only ITD, the other containing only interaural level differences (ILD) due to headshadow. Using this material, speech reception thresholds (SRT) for sentences in noise were determined for a group of normal-hearing subjects. Results show that (1) for noise azimuths between 30 degrees and 150 degrees, the gain due to ITD lies between 3.9 and 5.1 dB, while the gain due to ILD ranges from 3.5 to 7.8 dB, and (2) ILD decreases the effectiveness of binaural unmasking due to ITD (on the average, the threshold shift drops from 4.6 to 2.6 dB). In a second experiment, also conducted with normal-hearing subjects, similar stimuli were used, but now presented monaurally or with an overall 20-dB attenuation in one channel, in order to simulate hearing loss. In addition, SRTs were determined for noise with fixed ITDs, for comparison with the results obtained with head-induced (frequency dependent) ITDs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Observer weighting strategies in interaural time-difference discrimination and monaural level discrimination for a multi-tone complex

Two experiments measured listeners' abilities to weight information from different components in a complex of 553, 753, and 953 Hz. The goal was to determine whether or not the ability to adjust perceptual weights generalized across tasks. Weights were measured by binary logistic regression between stimulus values that were sampled from Gaussian distributions and listeners' responses. The first task was interaural time discrimination in which listeners judged the laterality of the target component. The second task was monaural level discrimination in which listeners indicated whether the level of the target component decreased or increased across two intervals. For both experiments, each of the three components served as the target. Ten listeners participated in both experiments. The results showed that those individuals who adjusted perceptual weights in the interaural time experiment could also do so in the monaural level discrimination task. The fact that the same individuals appeared to be analytic in both tasks is an indication that the weights measure the ability to attend to a particular region of the spectrum while ignoring other spectral regions.     

Lateral-position-based models of interaural discrimination

This letter investigates the hypothesis that lateral position is the only cue available for interaural discrimination experiments using 500-Hz stimuli. The discussion of this hypothesis is in the context of comparisons of the experimental data to predictions of the "position-variable" model of binaural interaction. The model predicts the mean and variance of the subjective lateral position of stimuli used in the discrimination experiments, assuming that discrimination performance is based on optimal processing of this subjective position. To the extent that the laterality predictions of the model are accurate, data that are inconsistent with its predictions would also be problematical for any model based on the subjective laterality of a single binaural image. The predictions (at least qualitatively) describe much of the observed experimental data, including a number of results that have not been addressed by any previous theory. Nevertheless, the observed performance is significantly better than the corresponding predictions for three types of experiments in which the utility of the position cue has been eliminated by experimental design. We believe that our results indicate that changes in lateral position are the primary cue in most interaural discrimination experiments, but that secondary attributes of the perceptual images can be useful when performance based on position alone would be poor.     

Forward-masked monaural and interaural intensity discrimination

Intensity-discrimination thresholds were measured for a 25-ms, 6-kHz pure tone for pedestal levels from 40 to 90 dB sound pressure level (SPL) with and without a forward masker (100-ms narrowband Gaussian noise, N(0)=70 dB). When the masker was present, the masker and probe were separated by 100 ms of silence. Unmasked and masked thresholds were measured in a two-interval monaural procedure and, separately, in a single-interval interaural procedure in which the pedestal and incremented pedestals were presented simultaneously to opposite ears. While the monaural thresholds were elevated markedly by the forward masker for mid-level pedestals, interaural thresholds were nearly unaffected by the masker across pedestal levels. The results argue against the notion that the monaural elevation in forward-masked thresholds is due to degraded encoding of intensity information at early stages of auditory processing.     

Rollover effect of signal level on vowel formant discrimination

The goal of this study was to measure the ability of normal-hearing listeners to discriminate formant frequency for vowels in isolation and sentences at three signal levels. Results showed significant elevation in formant thresholds as formant frequency and linguistic context increased. The signal level indicated a rollover effect, especially for F2, in which formant thresholds at 85 dB SPL were lower than thresholds at 70 or 100 dB SPL in both isolated vowels and sentences. This rollover level effect could be due to reduced frequency selectivity and forward/backward masking in sentence at high signal levels for normal-hearing listeners.     

Lateralization discrimination of interaural time delays in four-pulse sequences in electric and acoustic hearing

This study examined the sensitivity of four cochlear implant (CI) listeners to interaural time difference (ITD) in different portions of four-pulse sequences in lateralization discrimination. ITD was present either in all the pulses (referred to as condition Wave), the two middle pulses (Ongoing), the first pulse (Onset), the last pulse (Offset), or both the first and last pulse (Gating). All ITD conditions were tested at different pulse rates (100, 200, 400, and 800 pulses/s pps). Also, five normal hearing (NH) subjects were tested, listening to an acoustic simulation of CI stimulation. All CI and NH listeners were sensitive in condition Gating at all pulse rates for which they showed sensitivity in condition Wave. The sensitivity in condition Onset increased with the pulse rate for three CI listeners as well as for all NH listeners. The performance in condition Ongoing varied over the subjects. One CI listener showed sensitivity up to 800 pps, two up to 400 pps, and one at 100 pps only. The group of NH listeners showed sensitivity up to 200 pps. The result that CI listeners detect ITD from the middle pulses of short trains indicates the relevance of fine timing of stimulation pulses in lateralization and therefore in CI stimulation strategies.     

Effects of interaural time differences in fine structure and envelope on lateral discrimination in electric hearing

Bilateral cochlear implant (CI) listeners currently use stimulation strategies which encode interaural time differences (ITD) in the temporal envelope but which do not transmit ITD in the fine structure, due to the constant phase in the electric pulse train. To determine the utility of encoding ITD in the fine structure, ITD-based lateralization was investigated with four CI listeners and four normal hearing (NH) subjects listening to a simulation of electric stimulation. Lateralization discrimination was tested at different pulse rates for various combinations of independently controlled fine structure ITD and envelope ITD. Results for electric hearing show that the fine structure ITD had the strongest impact on lateralization at lower pulse rates, with significant effects for pulse rates up to 800 pulses per second. At higher pulse rates, lateralization discrimination depended solely on the envelope ITD. The data suggest that bilateral CI listeners benefit from transmitting fine structure ITD at lower pulse rates. However, there were strong interindividual differences: the better performing CI listeners performed comparably to the NH listeners.     

Buildup and breakdown of echo suppression for stimuli presented over headphones-the effects of interaural time and level differences

The current study investigates buildup and breakdown of echo suppression for stimuli presented over headphones. The stimuli consisted of pairs of 120-micros clicks. The leading click (lead) and the lagging click (lag) in each pair were lateralized on opposite sides of the midline by means of interaural level differences (ILDs) of +/-10 dB or interaural time differences (ITDs) of +/-300 micros. Echo threshold was measured with an adaptive one-interval, two-alternative, forced-choice procedure with a subjective decision criterion, in which listeners had to report whether they heard a single, fused auditory event on one side of the midline, or two separate events on both sides. In the control conditions, referred to as the "single" conditions, echo threshold was measured for a single click pair, the test pair, presented in isolation. In addition to the control conditions, two kinds of test conditions were investigated, in which the test pair was preceded by 12 identical conditioning pairs: in the "same" conditions, the interaural configuration (ILDs or ITDs) of the conditioning pairs was identical to that of the test pair; in the "switch" conditions, the interaural configuration of lead and lag was reversed between the conditioning pairs and the test pair, in order to produce a switch in the lateralizations of the stimuli between the conditioning train and the test pair. No matter whether the lateralization of the clicks was produced by ILDs or by ITDs, most listeners experienced a buildup of echo suppression in the "same" conditions, manifested by a prolongation of echo threshold relative to the respective "single" conditions. However, the breakdown of echo suppression was much stronger in the ILD-switch than in the ITD-switch conditions. In five out of six listeners, the ITD switch had hardly any effect on echo threshold, although the ITDs (+/-300 micros) produced roughly the same degree of lateral displacement as the ILDs (+/-10 dB). These results suggest that the dynamic processes in echo suppression operate differentially in pathways responsible for the processing of interaural time and level differences.     

Localization by interaural time difference (ITD): effects of interaural frequency mismatch.

A commonly accepted physiological model for lateralization of low-frequency sounds by interaural time delay (ITD) stipulates that binaural comparison neurons receive input from frequency-matched channels from each ear. Here, the effects of hypothetical interaural frequency mismatches on this model are reported. For this study, the cat's auditory system peripheral to the binaural comparison neurons was represented by a neurophysiologically derived model, and binaural comparison neurons were represented by cross-correlators. The results of the study indicate that, for binaural comparison neurons receiving input from one cochlear channel from each ear, interaural CF mismatches may serve to either augment or diminish the effective difference in ipsilateral and contralateral axonal time delays from the periphery to the binaural comparison neuron. The magnitude of this increase or decrease in the effective time delay difference can be up to 400 microseconds for CF mismatches of 0.2 octaves or less for binaural neurons with CFs between 250 Hz and 2.5 kHz. For binaural comparison neurons with nominal CFs near 500 Hz, the 25-microsecond effective time delay difference caused by a 0.012-octave CF mismatch is equal to the ITD previously shown to be behaviorally sufficient for the cat to lateralize a low-frequency sound source.     

The influence of interaural stimulus uncertainty on binaural signal detection

This paper investigated the influence of stimulus uncertainty in binaural detection experiments and the predictions of several binaural models for such conditions. Masked thresholds of a 500-Hz sinusoid were measured in an NrhoSpi condition for both running and frozen-noise maskers using a three interval, forced-choice (3IFC) procedure. The nominal masker correlation varied between 0.64 and 1, and the bandwidth of the masker was either 10, 100, or 1,000 Hz. The running-noise thresholds were expected to be higher than the frozen-noise thresholds because of stimulus uncertainty in the running-noise conditions. For an interaural correlation close to +1, no difference between frozen-noise and running-noise thresholds was expected for all values of the masker bandwidth. These expectations were supported by the experimental data: for interaural correlations less than 1.0, substantial differences between frozen and running-noise conditions were observed for bandwidths of 10 and 100 Hz. Two additional conditions were tested to further investigate the influence of stimulus uncertainty. In the first condition a different masker sample was chosen on each trial, but the correlation of the masker was forced to a fixed value. In the second condition one of two independent frozen-noise maskers was randomly chosen on each trial. Results from these experiments emphasized the influence of stimulus uncertainty in binaural detection tasks: if the degree of uncertainty in binaural cues was reduced, thresholds decreased towards thresholds in the conditions without any stimulus uncertainty. In the analysis of the data, stimulus uncertainty was expressed in terms of three theories of binaural processing: the interaural correlation, the EC theory, and a model based on the processing of interaural intensity differences (IIDs) and interaural time differences (ITDs). This analysis revealed that none of the theories tested could quantitatively account for the observed thresholds. In addition, it was found that, in conditions with stimulus uncertainty, predictions based on correlation differ from those based on the EC theory.     

Sensitivity to brief changes of interaural time and interaural intensity

The purpose of this study was to measure listeners' abilities to detect brief changes in interaural temporal disparities (ITDs) or interaural intensitive disparities (IIDs) conveyed by bursts of noise (probes) temporally and symmetrically flanked by segments of diotic or uncorrelated noise. Thresholds were measured using a four-interval, two-alternative, forced-choice adaptive task and the total duration of the bursts of noise was either 20, 40, or 100 ms. Probes were temporally centered within each burst and the durations of the probes ranged from 2 to 100 ms, depending upon the duration of the (longer) total burst of noise within which they were embedded. The results indicate that, for a given total duration of noise, there is a rapid decrease in threshold ITD or threshold IID as the duration of the probe is increased so that it occupies a larger portion of the total burst of noise. Mathematical analyses revealed that both threshold ITDs and threshold IIDs could be well accounted for by assuming that the listener processes both types of binaural cues via a single, symmetric, double-exponential temporal window. Interestingly, the shapes of the temporal windows that fit the data obtained from the human listeners resemble the shapes of the temporal windows derived by Wagner [H. Wagner, J. Comp. Physiol. A 169, 281-289 (1991)], who studied the barn owl. The time constants and relative weightings yielded temporal window functions that heavily emphasize information occurring within the very temporal center of the window. This temporal window function was found to be generalizable in the sense that it also accounts for classic data reported by Grantham and Wightman [D.W. Gratham and F.L. Wightman, J. Acoust. Soc. Am. 63, 511-523 (1978)] concerning sensitivity to dynamically changing interaural disparities.     

Test of a model of auditory object formation using intensity and interaural time difference discrimination.

A model of auditory object formation and an experimental evaluation of the model are described. Specifically, predictions for intensity discrimination and interaural time difference discrimination for the central component of a three-component harmonic complex are evaluated empirically. The onset time of the central, target component is varied relative to the onset times of the remaining, interferer components in order to vary the degree of fusion (versus perceptual segregation) of the target and the interferers. The model, which is based on the idea of attenuation of the components in the nonattended auditory image (in the case of segregated images), predicts lower sensitivity to information at the target component for the fused versus segregated target, and equal sensitivities for completely segregated targets and targets presented in isolation. Results are presented for four subjects with component frequencies of 400, 600, and 800 Hz and with onset time differences of 0 or 250 ms. The target duration was always 100 ms and offset times were the same for all components. The subjective results were as expected, with synchronous onsets yielding one sound object, and asynchrony of the central component yielding two sound objects. Also, the empirical results on interference in the synchronous case were in qualitative agreement with the above predictions. However, significantly more interference was found than was predicted for both synchronous and asynchronous conditions. In fact, the amount of interference found contradicts the simple attenuation model of object formation.